This invention relates to instruments for detecting acceleration and for producing signals in response thereto.
In various vehicles, as well as in other contexts, an instrument may be needed for sensing acceleration in one or more directions and for producing signals indicative of acceleration. Such an instrument may be used to activate some mechanism which is to be operated while a condition of acceleration exists. Considering a specific example, vehicles designed to travel over rough or uneven terrain often have wheels attached to the vehicle frame through resilient suspension systems to allow a wheel to travel over an obstacle without the upward motion being fully transmitted to the vehicle frame. This shock absorbing effect can be enhanced if a fluid cylinder is connected between the wheel and frame and if means are provided for sensing the vertical acceleration which accompanies passage of the wheel over an obstacle. The acceleration signal can be caused to actuate the fluid cylinder to forcibly lift the wheel relative to the frame enabling overriding of the obstacle with reduced upward movement of the frame itself. In other instances, typically found in earthworking vehicles, the operator's seat may have a fluid suspension in which fluid pressure is momentarily varied when upward or downward acceleration of the vehicle is sensed in order to reduce jarring of the operator when traveling over rough terrain. Many other forms of apparatus are known in which it is necessary to sense acceleration and to actuate some mechanism in response thereto.
The acceleration sensors heretofore employed for such purposes typically include a mass or weight suspended or supported by spring means. Due to inertial effects, any sudden acceleration of the structure surrounding the weight is accompanied by a lag in the movement of the weight itself. This relative movement between the surrounding structure and the weight is caused to actuate a slide valve which controls a fluid flow that in turn actuates a fluid motor or some other mechanism. Accelerometers of this kind are subject to the several difficulties inherent in the use of mechanical mechanism for performing sensitive detection functions. Such devices are necessarily bulky and are costly if manufactured to exhibit maximum precision. Sensitivity is limited by the friction between moving parts and there is a considerable risk of malfunctions from seizing, wearing and breakage of mechanical elements. Further, spring mass systems have inherent resonances which result in poor frequency response. In other words, a given spring mass system will oscillate much more strongly in response to acceleration at one particular rate or at harmonics thereof while being much less sensitive to acceleration at other rates.
To avoid the problems of mechanical accelerometers, fluidic devices have heretofore been designed in which acceleration is sensed from the deflection of a jet of fluid traveling across an open gap between a nozzle and one or more receiver ports. Jet deflection from acceleration causes pressure and flow changes at the receiver ports which are detected and amplified by fluidic circuit elements. As heretofore constructed, fluidic accelerometers have been subject to signal error from certain causes. For example, if such devices are operated in a tilted condition, gravitational deflection of the jet is changed with a resultant change in the output signal similar to what is produced by acceleration. Changes in the viscosity of the jet fluid from temperature changes may also cause an erroneous output signal by altering jet velocity and thereby altering gravitational drop of the jet in passage across the gap.